Among the mechanisms through which the cells of an organism communicate with each other and obtain information and stimuli from their environment is through cell membrane receptor molecules expressed on the cell surface. Many such receptors have been identified, characterized, and sometimes classified into major receptor superfamilies based on structural motifs and signal transduction features. The receptors are a first essential link for translating an extracellular signal into a cellular physiological response.
Receptors on neurons are particularly important in the development of the nervous system during embryogenesis. The neurons form connections with target cells during development through axonal extension of the neurons toward the target cells in a receptor-mediated process. Axons and dendrites have a specialized region of their distal tips known as the growth cone. Growth cones enable the neuron to sense the local environment through a receptor-mediated process and direct the movement of the axon or dendrite of the neuron toward the neuron's target cell. This process is known as elongation. Growth cones can be sensitive to several guidance cues, for example, surface adhesiveness, growth factors, neurotransmitters and electric fields. The guidance of growth at the cone depends on various classes of adhesion molecules, intercellular signals, as well as factors that stimulate and inhibit growth cones.
Interestingly, damaged neurons do not elongate in the central nervous system (CNS) following injury due to trauma or disease, whereas axons in the peripheral nervous system (PNS) regenerate readily. The fact that damaged CNS neurons fail to elongate is not due to an intrinsic property of CNS axons, but rather due to the CNS environment that is not permissive for axonal elongation. Classical grafting experiments by Aguayo and colleagues (e.g., Richardson et al., (1980) Nature 284, 264–265) demonstrated that CNS axons can in fact elongate over substantial distances within peripheral nerve grafts, and that CNS myelin inhibits CNS axon elongation. Therefore, given the appropriate environment, CNS axons can regenerate, implying that CNS axonal injury can potentially be addressed by appropriate manipulation of the CNS environment.
The absence of axon regeneration following injury can be attributed to the presence of axon growth inhibitors. These inhibitors are predominantly associated with myelin and constitute an important barrier to regeneration. Axon growth inhibitors are present in CNS-derived myelin and the plasma membrane of oligodendrocytes that synthesize myelin in the CNS (Schwab et al., (1993) Annu. Rev. Neurosci. 16, 565–595). Myelin-associated inhibitors appear to be a primary contributor to the failure of CNS axon regeneration in vivo after an interruption of axonal continuity, whereas other non-myelin associated axon growth inhibitors in the CNS may play a lesser role. These inhibitors block axonal regeneration following neuronal injury due to trauma, stroke or viral infection.
Numerous myelin-derived axon growth inhibitors have been characterized (see, for review, David et al., (1999) WO995394547; Bandman et al., (1999) U.S. Pat. No. 5,858,708; Schwab, (1996) Neurochem. Res. 21, 755–761). Several components of CNS white matter, NI35, NI250 (Nogo) and Myelin-associated glycoprotein (MAG), which have inhibitory activity for axonal extension, have been described as well (Schwab et al., (1990) WO9005191; Schwab et al., (1997) U.S. Pat. No. 5,684,133). In particular, Nogo is a 250 kDa myelin-associated axon growth inhibitor that was originally characterized based on the effects of the purified protein in vitro and monoclonal antibodies that neutralize the protein's activity (Schwab (1990) Exp. Neurol. 109, 2–5). The Nogo cDNA was first identified through random analysis of brain cDNA and had no suggested function (Nagase et al., (1998) DNA Res. 5, 355–364). The identification of this Nogo cDNA as the cDNA encoding the 250 kDa myelin-associated axon growth inhibitor was discovered only recently (GrandPre et al., (2000) Nature 403, 439–444; Chen et al., (2000) Nature 403, 434–439; Prinjha at al., (2000) Nature 403, 383–384).
Importantly, Nogo has been shown to be the primary component of CNS myelin responsible for inhibiting axonal elongation and regeneration. Nogo's selective expression by oligodendrocytes and not by Schwann cells (the cells that myelinate P.S. axons) is consistent with the inhibitory effects of CNS myelin, in contrast to P.S. Myelin (GrandPre et al., (2000) Nature 403, 434–439). In culture, Nogo inhibits axonal elongation and causes growth cone collapse (Spillmann et al., (1998) J. Biol. Chem. 272, 19283–19293). Antibodies (e.g., IN-1) against Nogo have been shown to block most of the inhibitory action of CNS myelin on neurite growth in vitro (Spillmann et al., (1998) J. Biol. Chem. 272:19283–19293). These experiments indicate that Nogo is the main component of CNS myelin responsible for inhibition of axonal elongation in culture. Furthermore, in vivo, the IN-1 antibody has been shown to enhance axonal regeneration after spinal cord injury, resulting in recovery of behaviors such as contact placing and stride length (Schnell and Schwab (1990) Nature 343, 269–272; Bregman et al., (1995) Nature 378, 498–501). Thus, there is substantial evidence that Nogo is a disease-relevant molecular target. Agents that interfere with the binding of Nogo to its receptor would be expected to improve axonal regeneration in clinical states in which axons have been damaged, and improve patient outcome.
Modulation of Nogo has been described as a means for treatment of regeneration for neurons damaged by trauma, infarction and degenerative disorders of the CNS (Schwab et al., (1994) WO9417831: Tatagiba et al., (1997) Neurosurgery 40, 541–546) as well as malignant tumors in the CNS such as glioblastoma (Schwab et al., (1993) U.S. Pat. No. 5,250,414); Schwab et al., (2000) U.S. Pat. No. 6,025,333).
Antibodies which recognize Nogo have been suggested to be useful in the diagnosis and treatment of nerve damage resulting from trauma, infarction and degenerative disorders of the CNS (Schnell & Schwab, (1990) Nature 343, 269–272; Schwab et al., (1997) U.S. Pat. No. 5,684,133). For CNS axons, there is a correlation between the presence of myelin and the inhibition of axon regeneration over long distances (Savio and Schwab (1990) Proc. Natl. Acad. Sci. 87, 4130–4133; Keirstead et al., (1992) Proc. Natl. Acad Sci. 89, 11664–11668). After Nogo is blocked by antibodies, neurons can again extend across lesions caused by nerve damage (Schnell and Schwab (1990) Nature 343, 269–272).